CORRELATION OF AIRBORNE ELECTROMAGNETIC GEOPHYSICAL DATA WITH KARST VADOSE PERMEABILITY

Quantifying recharge to karstic aquifers is a complex hydrogeologic objective, requiring detailed conceptual models and the ability to measure discrete and diffuse mechanisms of infiltration. The challenge of scaling local observations of infiltration to regional estimates often limits accurate modeling of karst aquifer recharge. Moreover, it is difficult to accurately estimate the distribution and density of karstic features that influence upland (diffuse) recharge rates. At Camp Bullis, a U.S. military facility north of San Antonio, Texas, two unique datasets exist that could significantly improve the ability to spatially resolve the degree karst features enhance diffuse recharge rates for the Edwards and Trinity aquifer system. The first is the result of 17 years of detailed, ground-based karst feature surveys, covering every square meter of the 11,400 ha and including extensive excavations to determine the full extent of each located feature. These efforts identified over 1100 karst features in nine separate hydro-stratigraphic units. The second dataset is from an airborne electromagnetic survey that provides detailed frequency-domain EM geophysical measurements with 100 m and 200 m flight line spacing.

The EM data were inverted using EM1DFM to yield nearly 218000 resistivity-depth profiles over the karst feature survey area. Resistivity-depth profile curves, which exhibit many different shape characteristics (minima, maxima, peaks, etc.) resulting from modeling electrical resistivity variations with depth, were used to assess over 700 karst features located 50 m or less from the nearest flight line. Statistical distribution analyses of the profiles that are in closest proximity to mapped features indicate a significant bias that is not evident in random close-proximity points or in the survey area as a whole. Thus, a possibly significant correlation exists between known karst features and the specific pattern of inverted EM profiles. Identification and gridding of profile classifications may provide a useful tool to quantitatively model and map vadose zone permeability in karst. These types of data can greatly improve the ability to model recharge and account for complex heterogeneity.